This application examines the cause for age-associated neuromuscular fatigue in tasks that are more functionally relevant to daily activities and that are important for maintenance of independence in old adults within an aging American population. This proposal brings a new and innovative approach to understanding fatigability of the neuromuscular system in old adults by examining functionally relevant tasks that require control of a load with limited limb support and maintenance of power for a dynamic task. Recent studies indicate that older adults experience greater muscle fatigue than young adults during these more functionally relevant tasks. However, the causes of the increase in age-related muscle fatigue in old adults during submaximal dynamic and postural contractions with limited limb support are not understood. Based on preliminary data we hypothesize that neural mechanisms are responsible for greater fatigue exhibited by old adults compared with young adults for submaximal fatiguing contractions that require supporting an inertial load. First, we examine the mechanisms for age-related changes in muscle fatigue of dynamic and postural tasks that involve supporting an inertial load. Specifically, we will compare the time to task failure between young and old adults for a postural isometric and dynamic task and establish the association between the age-related fatigue and functional ability in old adults (Aim 1). We will also determine neural mechanisms that contribute to time to task failure for these tasks in young and old adults (Aim 2). To accomplish Aim 2, we will use transcranial magnetic stimulation to examine corticospinal excitability before, during and after fatigue and this will be quantified as the size of the motor evoked potential normalized to the changes in neuromuscular propagation (M wave). By understanding the magnitude and mechanisms that contribute to age-related fatigue during functionally relevant tasks, we can identify appropriate and simple rehabilitative techniques to improve the increased fatigue experienced by old adults. Our preliminary data suggests that 'practice' of a fatiguing task will improve performance via neural adaptations. Therefore, we will determine the change in time to task failure and corticospinal excitability with practice of a postural isometric and dynamic task performed by young and old adults (Aim 3). This last aim will evaluate the efficacy of 'practice' as a rehabilitative technique, to ameliorate neuromuscular function deficits and minimize impairments in old adults, and also to provide evidence of age-related neural impairments in performance. [unreadable] [unreadable]